More logicial order. Move space saving step to just before it is used.

Modules/_collectionsmodule.c

@@ -386,6 +386,13 @@ deque_extend(dequeobject *deque, PyObject *iterable)
         return result;
     }
 
+    it = PyObject_GetIter(iterable);
+    if (it == NULL)
+        return NULL;
+
+    if (maxlen == 0)
+        return consume_iterator(it);
+
     /* Space saving heuristic.  Start filling from the left */
     if (Py_SIZE(deque) == 0) {
         assert(deque->leftblock == deque->rightblock);
@@ -394,13 +401,6 @@ deque_extend(dequeobject *deque, PyObject *iterable)
         deque->rightindex = 0;
     }
 
-    it = PyObject_GetIter(iterable);
-    if (it == NULL)
-        return NULL;
-
-    if (maxlen == 0)
-        return consume_iterator(it);
-
     iternext = *Py_TYPE(it)->tp_iternext;
     while ((item = iternext(it)) != NULL) {
         if (deque_append_internal(deque, item, maxlen) < 0) {
@@ -433,6 +433,13 @@ deque_extendleft(dequeobject *deque, PyObject *iterable)
         return result;
     }
 
+    it = PyObject_GetIter(iterable);
+    if (it == NULL)
+        return NULL;
+
+    if (maxlen == 0)
+        return consume_iterator(it);
+
     /* Space saving heuristic.  Start filling from the right */
     if (Py_SIZE(deque) == 0) {
         assert(deque->leftblock == deque->rightblock);
@@ -441,13 +448,6 @@ deque_extendleft(dequeobject *deque, PyObject *iterable)
         deque->rightindex = BLOCKLEN - 2;
     }
 
-    it = PyObject_GetIter(iterable);
-    if (it == NULL)
-        return NULL;
-
-    if (maxlen == 0)
-        return consume_iterator(it);
-
     iternext = *Py_TYPE(it)->tp_iternext;
     while ((item = iternext(it)) != NULL) {
         if (deque_appendleft_internal(deque, item, maxlen) < 0) {